This invention relates to automatic identification targets having radio frequency taggants formed from thin metal objects or dipoles.
Automatic identification systems are widely used to input data into computer systems and to control operation of equipment. Radio frequency operated systems are often used in automatic identification applications where identification of an object is to be made without contact and where the application may not permit line-of-sight communication between the object and a sensor. Radio frequency automatic identification (xe2x80x9cRF/AIDxe2x80x9d) systems are based on xe2x80x9ctargetsxe2x80x9d which generally function as transponders. Upon receipt of a radio frequency interrogating signal, the target responds by producing a detectable radio frequency response signal. Such targets have taken the form of tags or labels which are affixed to an object to be identified. As used herein, a xe2x80x9ctargetxe2x80x9d is any radio frequency responsive means which may be attached to, printed upon, embedded in, or otherwise associated with an object to be identified. Although the term xe2x80x9cradio frequencyxe2x80x9d is used herein because that is the region of the electromagnetic spectrum which is most frequently used for such automatic identification, it will be understood that the invention embraces any electromagnetic radiation. Automatic identification systems are presently used or contemplated for use in a wide variety of applications for monitoring of people, animals, locations, and things. Such applications include material handling, such as automatic storage and retrieval; cargo handling, such as baggage sorting and tracking; asset management, such as of rental vehicles or for retail theft control; identification of persons, such as for facility access control or patient tracking; and identification of animals, such as for automatic feeding.
One major attribute of presently available RF/AID systems which limits their use is that the target cost is substantial. Situations in which this high cost can be justified include those where targets are removed from an item after purchase and reused on other items, those where the item to be identified is costly or important, and those where an item is likely to be misappropriated absent some effective control system. Another attribute which restricts use of present RF/AID systems is the target size. Targets are typically several inches long, which inhibits their use where small items are to be identified or where it is desired that the target be unobtrusive.
Both of these attributes result in large part from the structure typically employed for RF/AID targets and the frequency at which they operate. Such targets typically contain an antenna for receiving an interrogating RF signal; radio frequency processing means for determining whether an interrogating signal has been received; and radio frequency transmitting means responsive to the processing means for transmitting a detectable RF response signal from the target. Present systems typically operate at fairly low frequencies.
One application for which existing RF/AID techniques have not been used, due to the cost, size, and limited information content of prior art targets, is identification of documents including currency, financial instruments such as credit cards, and personal identification instruments such as a driver""s license or passport. Counterfeiting of currency has become a major problem, on scales and using technology ranging from color photocopying to altering of genuine currency printing operations. Credit card fraud, involving magnetic programming of counterfeit cards or reprogramming of genuine ones, has become epidemic. Existing techniques for identifying such items are obviously deficient. Ordinary business documents would desirably be protectable from unauthorized copying or use and verifiable as original or genuine, but no effective means for doing so are available.
The present invention is directed to a new system for RF/AID which avoids the foregoing drawbacks of the prior art and is suitable for use in a wide variety of applications which were not possible using prior art systems.
It is therefore an object of the present invention to provide a low cost system for producing radio frequency responsive targets which are suitable for identifying and authenticating a wide variety of objects including currency, financial instruments such as credit cards, and personal identification instruments such as a driver""s license or passport.
It is a further object of the present invention to provide a low cost, reliable, high speed, system for reading a radio frequency responsive target associated with an object and thereby verifying the authenticity of the object.
These and other objects and advantages of the invention will become more fully apparent from the description and claims which follow or may be learned by the practice of the invention.
The present invention is directed to a method for forming a radio frequency responsive target formed of a pattern of thin dipoles which produce a composite analog radio frequency signal in response to an interrogating signal. A first metallic film layer is deposited on top of a non-conductive substrate. The pattern of thin dipoles is generated such that each of the thin dipoles has a position and an angular orientation within the pattern of thin dipoles. An etchant resistant pattern is deposited on top of the first metallic film layer, the etchant resistant pattern corresponds to a photographic image of the pattern of thin dipoles. A second metallic layer is applied on top of the first metallic film layer. The second metallic layer occupies areas on the first metallic film layer where the etchant resistant pattern is absent, and the second metallic layer has a thickness equal to the thickness of the etchant resistant pattern. The etchant resistant pattern is removed in order to expose portions of the first metallic film layer. The second metallic layer and the portions of the first metallic film layer are etched simultaneously until the portions of the first metallic film layer have been removed from the non-conductive substrate.